Delete docs/v0.3.0-design.md and strip all references to it

The design doc and its followups doc are no longer needed as a live
reference now that the v0.3.0 redesign is implemented — comments and
docstrings across the codebase cited it extensively (file path, "design
doc §X.Y", "decision N", or bare "§X.Y" section numbers) as design
rationale. Removed docs/ and edited every citing comment/docstring to
drop the now-dangling reference while keeping the substantive
explanation next to it. CLAUDE.md's v0.3.0 roadmap bullet loses its
trailing pointer to the deleted file.

Verified: no remaining "docs/v0.3.0", "design doc", "decision N", or
"§N.N" references (repo-wide grep); ruff and ty clean; full test suite
on the heaviest-touched modules (network, sample, rollout, migration,
config, train) passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

giant/data/setup_cache.py

@@ -104,7 +104,7 @@ def normalizer_key(
 ) -> str:
     # .6g avoids float-repr drift (e.g. 0.1 vs 0.10000000000000002) causing
     # spurious cache misses between runs with the "same" val_fraction. The two
-    # conditioning axes are independent (docs/v0.3.0-design.md §3.1) and both
+    # conditioning axes are independent and both
     # affect which cond_cont columns are computed for real vs. zero-filled
     # (giant.data.transforms._physical_cond_columns), so both must be part of
     # the key or two mixed-axis runs could collide on the same cache entry.
@@ -114,7 +114,7 @@ def normalizer_key(
     )
 
 
-# Top-N-map axes (docs/v0.3.0-design.md §8): "pdg" keys match pdg_map's int
+# Top-N-map axes: "pdg" keys match pdg_map's int
 # keys (shared by conditioning.particle.type="onehot" and
 # stage2_model.particle_type.target="onehot" — one map for both), "material"
 # keys match mat_map's str keys.
@@ -190,7 +190,7 @@ class SetupCache:
     proc_maps: dict[int, dict[str, int]] = field(default_factory=dict)
     normalizers: dict[str, NormalizerEntry] = field(default_factory=dict)
     topn_maps: dict[str, TopNMap] = field(default_factory=dict)
-    """Keyed by `topn_key(axis, n_classes)` — see docs/v0.3.0-design.md §8."""
+    """Keyed by `topn_key(axis, n_classes)`."""
 
     @classmethod
     def empty(cls, files: list[Path]) -> "SetupCache":